Well, technically, since this not a criminal case, nobody has to 'prove' anything. One side just has to have a preponderance of evidence. So far, the analysis of the code shows that only a fraction of a percentage of it is close enough to Sun/Oracle's to warrant looking more closely at it.
And while it offers no direct evidence one way or another, Google's intention from the beginning was to have the source code to Android published and would expect many, many eyeballs on their source code. If nothing else, it's inconceivable to me that the strategy at Google was, "let's clean-room implement 99.5% of this code, but then directly copy this other 0.5% of the code in order to save us time and money." A single engineer being lazy, on the other hand? I could buy that, maybe.
Clean-room reimplementation is not a new or obscure activity in software. It's been done for many decades and the legal hoops to jump through to perform it are well known. Standard practice is to have anyone who contributes as much as a line of code to sign an affidavit stating that they had never so much as glanced at the source code to what is being reimplemented. I suspect that Google has quite a lot of evidence that the clean room implementation was done according to legal standards. The truth is, though, a lot of APIs behave in ways that provide very few reasonable code structures to accomplish its implementation and therefore it is to be expected that even a clean-room implementation will have more than a passing resemblance to the original.